Recently there has been widespread adoption of systems for wireless reading of coded information and similar from objects for reading known as RFID tags.
In such systems, a device to read coded information and similar from RFID tags is called an RFID reader/writer. An RFID tag has an IC memory which stores coded information, but is not provided with a power source, in order to enable miniaturization. Hence the supply of power is necessary in order to read coded information from the IC memory and transmit the coded information wirelessly to the RFID reader/writer.
When the RFID reader/writer reads coded information and similar from an RFID tag, an unmodulated continuous wave (CW) is transmitted to the RFID tag. The RFID tag receives the unmodulated continuous wave, and converts this into a current to receive a supply of power. This power is used to read coded information from the IC memory and to modulate the unmodulated continuous wave and return the modulated wave to the RFID reader/writer. By this means, the RFID reader/writer can read coded information or similar from an RFID tag.
FIG. 1 is a conceptual diagram of an example of the configuration of such an RFID reader/writer. In FIG. 1, an information read processing circuit 3 is connected to an antenna 1 via a coaxial cable 2. The antenna 1 has a plate-shape radiating conductor 10 which is positioned parallel to and opposing a grounded plate 12 by means of insulating supports 11a to 11d, of Teflon or another material.
In the example shown in FIG. 1, a configuration is employed in which air intervenes between the patch antenna (plate-shape radiating conductor) 10 and the grounded plate 12 by means of the insulating supports 11a to 11d; but a configuration is also possible in which an insulating plate of Teflon or similar intervenes. The plate-shape radiating conductor 10 further has an electromagnetic wave radiating window 13.
The transmission/reception portion of the information read processing circuit 3 is connected via a circulator 30 to the transmission amplifier(amp)SPA and to the reception amp RAP. Beyond the transmission amp SPA and reception amp RAP is connected a processing circuit, which however is not directly related to this invention, and so is omitted from drawings.
The feed point P of the plate-shape radiating conductor 10 and the circulator 30 are connected by the coaxial cable 2. The unmodulated continuous wave (CW) output from the transmission amp SPA passes through the coaxial cable 2, is supplied to the feed point, and is radiated from the plate-shape radiating conductor 10 toward the RFID tag. The unmodulated continuous wave (CW) is modulated and reflected by the RFID tag, and is received by the plate-shape radiating conductor 10, passes through the coaxial cable, is received by the information read processing circuit 3, and is received from the circulator 30 by the reception amp RPA.
Here, the characteristic impedance of the coaxial cable 2 is 50 Ω. If the impedance of the feed point P is different from the characteristic impedance of the coaxial cable 2, then the unmodulated continuous wave (CW) supplied from the transmission amp SPA is reflected at the feed point.
On the other hand, the RFID reader/writer receives a minute response signal from the RFID tag, and so reflection from the antenna 10 becomes an interference wave, and the sensitivity is lowered. In a normal antenna, even a reflection characteristic of approximately −10 dB is sufficient, but in an RFID reader/writer, a reflection characteristic of −20 dB or lower is desirable.
Various proposals have been made in the prior art with respect to improvement of the antenna reflection characteristics (for example, in Japanese Patent Publication No. 8-8446 and Japanese Patent Laid-open No. 2001-203529). In the invention described in Japanese Patent Publication No. 8-8446, as shown in the plane view of FIG. 2 and the cross-sectional view along line A-A′ in FIG. 3, a plate-shape radiating conductor 10 is positioned in opposition to a grounded plate 12, with a dielectric substrate 14 intervening. The position of placement of the feed point P from the center O of the plate-shape radiating conductor 10 is adjusted, and the central conductor 16 of the coaxial cable is connected to the feed point P, while the outer conductor 17 is connected to the grounded plate 12.
As one characteristic, protrusions 15 or cutouts (Japanese Patent Publication No. 8-8446, FIG. 3) are provided on the outer periphery of the plate-shape radiating conductor 10 at positions at prescribed angles from the feed point P of the plate-shape radiating conductor 10, and the sizes thereof are adjusted.
In the invention described in Japanese Patent Laid-open No. 2001-203529, as shown in FIG. 4, a radiating conductor 10 is formed having a cutout 9 in the substrate 20, and a slit 22 is further provided between the feed line 21 and radiating conductor 10. The antenna operating mode is obtained through the width and length of the slit 22, and by adjusting the length the desired impedance matching is obtained.
However, in methods to adjust the position of the feed point in such examples of the prior art, adjustment processing is not easily performed, and moreover there is the problem that the polarization states which occur change with the position of the feed point.